Had military training. He used the word "Klick" when describing how far Kate was when following Jack, Locke, and Sawyer for Michael.
The Danny that Ana Lucia dated. He knew Sawyer has sex with her and this is his motivation behind wanting to kill him.
Danny broke up with Ana Lucia sometime during the 4 months that she was off work. Pickett was married to Colleen. Therefore he wouldn't have cared if Sawyer and Ana Lucia slept together. Since Jack is the only person who knows about them, Pickett is unlikely to even know about it.
Was angry at Sawyer because he was told to beat Kate's love confession out of Sawyer.
Was angry with the 815 survivors in general, for killing Colleen. Took it out on Sawyer because they needed Jack to operate on Ben, and he didn't want to hurt Kate because she is a woman and/or might be needed to pursuade Jack to operate.
Was angry at Sawyer because he was jealous that Sawyer's lover (Kate) was still alive and well, while Pickett's lover (Colleen) was dead.
Tortured Sawyer not because he hated him but because it was a ploy to make Kate fall for him and have sex with him, resulting in her pregnancy.
Was actually rude to everyone, but had some reason for being nice to Michael.
Danny is ex-DHARMA. He was angry at Sawyer due to something that happened in DHARMA days, possibly not realizing that Sawyer was "LaFleur" but only thinking he looked like him. Sawyer would not understand this because it hadn't happened in his time yet.